Spray some lemon juice on the stain, or some hydrogen peroxide. I've had good luck with the lemon juice. I read once you can rub some salt after the lemon juice, but plain (non-diluted) lemon juice worked for me! I put the juice on and let it sit, then laundered it as normal. I scrubbed it a little, not a lot, just enough to get the lemon juice all over the stain.
I've also had good luck with Oxy-clean, but I've never tried it on underarm stains. Usually I mix up a cup, stick the part with the stain on it in the cup and let it sit. Then I throw it in the laundry. I hate scrubbing too! Who has time (or the knuckles) for that?